Pregnant women who sleep on their left side may be decreasing their risk of delivering a stillborn child, a retrospective case-control study suggested.

Action Points

Explain that a case-control study found that sleeping in any position other than on the left side on the night before delivery was associated with late stillbirth.

Point out that the study was considered hypothesis-generating, and the investigators called for confirmation in further studies.

Pregnant women who sleep on their left side may be decreasing their risk of delivering a stillborn child, a retrospective case-control study suggested.

Compared with women who slept on their left side on the last night before delivery, those who slept on their right side had an adjusted odds ratio for late stillbirth of 1.74 (95% CI 0.98 to 3.01), according to Tomasina Stacey, a PhD student, and colleagues from the University of Auckland in New Zealand.

And the odds ratio for stillbirth with supine sleeping on the night before fetal death was 2.54 (95% CI 1.04 to 6.18, P=0.005), the researchers reported online in BMJ.

However, an editorial accompanying the study advised caution in interpreting these findings.

"A forceful campaign urging pregnant women to sleep on their left side is not yet warranted," wrote Lucy C. Chappell, MD, of King's College London, and Gordon C.S. Smith, MBChB, of the University of Cambridge.

Little is known about maternal sleep patterns and the risk of stillbirth, although studies have suggested that sleeping supine may be associated with disordered breathing and lower maternal cardiac output.

Anatomical studies also suggest that lying supine or on the right side can increase the pressure of the uterus on the aorta and inferior vena cava, which can impede uterine blood flow.

To explore the possibility that maternal sleeping patterns and positions could influence the occurrence of stillbirth, Stacey and colleagues analyzed data from the Auckland Stillbirth Study, which included 155 women who had a stillbirth at 28 weeks or later and 310 matched controls from the same gestational period who had successful deliveries.

The prevalence of stillbirth among the cohort was 3.09 per 1,000 births, the researchers reported.

The study found no association with maternal snoring or with daytime sleepiness as measured on the Epworth sleepiness scale.

Univariate analysis found these significant associations with stillbirth:

Sleeping during the day during the last month, OR 1.78 (95% CI 1.18 to 2.68, P=0.006)

Sleeping more than eight hours at night, OR 1.83 (95% CI 1.14 to 2.94, P=0.02)

Getting up to use the bathroom infrequently on the last night of pregnancy (≤1 time), OR 1.55 (95% CI 1.04 to 2.30, P=0.03)

Using the bathroom infrequently during the last month and last week of pregnancy also were associated with stillbirth, with odds ratios of 1.44 (95% CI 0.97 to 2.14, P=0.07) and 1.62 (95% CI 1.09 to 2.41, P=0.02), respectively.

After adjustment for multiple potential confounding factors, analysis revealed that sleeping in any position other than on the left side doubled the risk of late stillbirth (OR 2.03, 95% CI 1.24 to 3.29), the researchers reported.

However, they pointed out that the absolute risk for stillbirth in developed countries is low and would remain small even without left-sided sleeping, at approximately 3.93 per 100.

Limitations of the study include its case-control design, which allows for the possibility of recall bias, uncertainty about the precise time of fetal demise, and a 72% recruitment rate for both cases and controls, which raises the possibility of selection bias.

The authors noted that this is the first study to find an association between maternal sleep and stillbirth, and called for additional research.

Editorialists Chappell and Smith emphasized the scope of the problem in the developing world, where 10 times more stillbirths occur than deaths from sudden infant death syndrome, which has been the focus of much attention.

But they argued that the design of this study, with multiple comparisons rather than a prespecified primary outcome measure, "means that it must be considered as a hypothesis-generating study rather than a hypothesis-testing one."

They echoed the authors' call for future research, and highlighted the potential for significant benefit.

"If these findings are validated in a future study, advice on sleep position is an intervention that would be relatively easy to implement. The message is appealing, perhaps partly because of resonance with the campaign on changing sleep position for infants, which led to a marked reduction in rates of sudden infant death," wrote Chappell and Smith.

The study was funded by Cure Kids, the Nurture Foundation, and the Auckland District Health Board Trust.

All authors and editorialists declared that they had no financial conflicts.

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